Percussion drill assembly

Abstract

A compressor piston divides a first compartment into two compression chambers, while a hammer piston divides a second compartment into two drive chambers, each of the compression chambers being connected to a respective one of the drive chambers to form a closed fluid system wherein reciprocation of the compressor piston causes cyclic compression and expansion of the fluid in the compression chambers and thus in the drive chambers, to effect a cyclic impacting of the hammer piston with a bit adapter connected to the drill bit. A mud motor rotates a shaft to drive an oscillator which reciprocates the compressor piston. The oscillator can comprise roller elements in the compressor piston in engagement with canted grooves in the shaft. While drilling mud drives the motor and then passes downwardly to flush the drill bit and the borehole, the drilling mud is isolated from the closed fluid system. The bit adapter slides axially, so that when the drill bit is not in contact with a borehole bottom, the bit adapter and the hammer piston move downwardly to a position where the two drive chambers are in direct communication such that the reciprocation of the compressor piston does not actuate the hammer piston. Each of the pistons is an annular piston having a bleed passageway between its chambers, permitting the chambers to equalize when the pistons are stationary. The superatmospheric pressure is such that the hammer piston reciprocates at a frequency within ±20% of natural resonant frequency.

We claim: 
     
       1. A percussion drill assembly for drilling a borehole in a formation, said drill assembly comprising: an elongated housing assembly having a first end adapted to removably connect said drill assembly to a drill string, and a second end adapted to receive a drill bit;   a compartment formed within said housing assembly and having a longitudinal axis;   a hammer piston positioned within said compartment for reciprocal motion within said compartment along the longitudinal axis of said compartment, said hammer piston dividing said compartment into a first chamber and a second chamber which are substantially fluidly isolated from each other within said compartment by the presence of said hammer piston;   a fluid compressor positioned within said housing assembly and having a first port in said first chamber and a second port in said second chamber, whereby said compressor and said compartment form a compressor fluid system;   a driver mounted in said housing assembly and connected to said compressor so as to drive said compressor to produce a high fluid pressure in said first port and a low fluid pressure in said second port during a first half cycle of operation of said compressor and to produce a low fluid pressure in said first port and a high fluid pressure in said second port during a second half cycle of operation of said compressor;   seals for sealing said compressor fluid system from fluid communication with any fluid received from the drill string, whereby said compressor fluid system is a closed fluid system;   wherein, when said drill assembly is being operated to impart an impact force to a drill bit, a high fluid pressure in said first chamber and a low fluid pressure in said second chamber causes a movement of said hammer piston toward said second chamber; and   wherein, when said drill assembly is being operated to impart an impact force to a drill bit, a low fluid pressure in said first chamber and a high fluid pressure in said second chamber causes a movement of said hammer piston toward said first chamber;   whereby a predetermined extent of movement of said hammer piston toward one of said first and second chambers can impart an impact force to a drill bit connected to said second end of said housing assembly while said drill assembly is being operated to impart an impact force to the drill bit.   
     
     
       2. A percussion drill assembly in accordance with claim 1, wherein said hammer piston is a free-floating piston. 
     
     
       3. A percussion drill assembly in accordance with claim 1, wherein said housing assembly comprises a bit adapter at said second end of said housing assembly for receiving a drill bit, said bit adapter having an anvil surface exposed to said compartment; and wherein said drill assembly further comprises a drill bit removably connected to said bit adapter;   whereby a predetermined extent of movement of said hammer piston in one of its directions of movement causes said hammer piston to strike said anvil surface and impart an impact blow to said bit adapter when said drill bit is in contact with a borehole bottom.   
     
     
       4. A percussion drill assembly in accordance with claim 3, wherein said driver comprises a fluid motor which is driven by a drilling fluid passed downwardly through a drill string to the drill assembly, and wherein the drilling fluid is exhausted from said fluid motor through said second end of said housing assembly and through said drill bit. 
     
     
       5. A percussion drill assembly in accordance with claim 3, wherein said drill bit adapter engages the remainder of said housing assembly so that rotation of said housing assembly by rotation of the drill string causes the drill bit adapter and the drill bit to rotate in a corresponding manner. 
     
     
       6. A percussion drill assembly in accordance with claim 3, wherein said bit adapter can slide axially with respect to the remainder of said housing assembly, whereby said bit adapter can move downwardly when the drill bit is not in contact with a borehole bottom; and wherein said housing assembly has a connector passageway which is blocked when said drill bit is in contact with a borehole bottom and which is unblocked when said drill bit moves downwardly as a result of the drill bit not being in contact with a borehole bottom, whereby said connector passageway provides fluid communication between said first and second chambers when said connector passageway is unblocked.   
     
     
       7. A percussion drill assembly in accordance with claim 6, wherein said drill bit adapter engages the remainder of said housing assembly so that rotation of said housing assembly by rotation of the drill string causes the drill bit adapter and the drill bit to rotate in a corresponding manner. 
     
     
       8. A percussion drill assembly in accordance with claim 1, wherein said driver comprises a fluid motor which is driven by a drilling fluid passed downwardly through a drill string to the drill assembly, and wherein the drilling fluid is exhausted from said fluid motor through said second end of said housing assembly. 
     
     
       9. A percussion drill assembly in accordance with claim 8, wherein said motor has a liquid inlet and a liquid outlet, said motor has a stator and a rotor positioned between said liquid inlet and said liquid outlet, said rotor is connected to a rotary shaft so that rotation of said rotor causes corresponding rotation of said rotary shaft, wherein the rotation of said rotary shaft drives said compressor, and wherein said liquid inlet of said motor is connected to an inlet passageway in said first end of said housing assembly so that liquid from a drill string flows through said inlet passageway and then flows between said stator and said rotor to said liquid outlet to effect rotation of said rotor with respect to said housing assembly, thereby rotating said rotary shaft and driving said compressor. 
     
     
       10. A percussion drill assembly in accordance with claim 1, wherein a second compartment is formed within said housing assembly, said second compartment having a longitudinal axis; wherein said compressor comprises a compressor piston positioned within the second compartment for reciprocal motion within said second compartment along the longitudinal axis of said second compartment, said compressor piston dividing said second compartment into a third chamber and a fourth chamber which are substantially fluidly isolated from each other within said second compartment by the presence of said compressor piston; wherein said first port provides fluid communication with said third chamber, and said second port provides fluid communication with said fourth chamber; and   wherein said driver is connected to said compressor piston to cause reciprocating movements of said compressor piston within said second compartment along the longitudinal axis of said second compartment.   
     
     
       11. A percussion drill assembly in accordance with claim 1, wherein said housing assembly further comprises: a bit adapter at said second end of said housing assembly for receiving a drill bit, wherein said bit adapter can slide axially with respect to the remainder of said housing assembly, whereby said bit adapter can move downwardly when the drill bit is not in contact with a borehole bottom; said bit adapter having an anvil surface exposed to said compartment; and   a connector passageway which is blocked when the drill bit is in contact with a borehole bottom and which is unblocked when the drill bit adapter moves downwardly as a result of the drill bit not being in contact with a borehole bottom, whereby said connector passageway provides fluid communication between said first and second chambers when said connector passageway is unblocked;   whereby a predetermined extent of movement of said hammer piston in a first one of its directions of movement causes said hammer piston to strike said anvil surface and impart an impact blow to said bit adapter when said drill bit is in contact with a borehole bottom; and   whereby a movement of said hammer piston in said first one of its directions of movement does not cause said hammer piston to strike said anvil surface and impart an impact blow to said bit adapter when said drill bit is not in contact with a borehole bottom.   
     
     
       12. A percussion drill assembly for drilling a borehole in a formation, said drill assembly comprising: an elongated housing assembly having a first end adapted to removably connect said drill assembly to a drill string, and a second end adapted to receive a drill bit;   a first compartment formed within said housing assembly and having a longitudinal axis;   a compressor piston positioned within said first compartment for reciprocal motion within said first compartment along the longitudinal axis of said first compartment, said compressor piston dividing said first compartment into a first chamber and a second chamber which are substantially fluidly isolated from each other within said first compartment by the presence of said compressor piston;   a driver mounted in said housing assembly and connected to said compressor piston to cause reciprocating movements of said compressor piston within said first compartment along the longitudinal axis of said first compartment;   a second compartment formed within said housing assembly and having a longitudinal axis;   a hammer piston positioned within said second compartment for reciprocal motion within said second compartment along the longitudinal axis of said second compartment, said hammer piston dividing said second compartment into a third chamber and a fourth chamber which are substantially fluidly isolated from each other within said second compartment by the presence of said hammer piston;   a first passageway providing fluid communication between said first chamber and said third chamber;   a second passageway providing fluid communication between said second chamber and said fourth chamber;   seals for sealing said first and second compartments and said first and second passageways from fluid communication with any fluid received from the drill string, whereby said first and second compartments and said first and second passageways constitute a closed fluid system;   each of said first, second, third, and fourth chambers, and said first and second passageways being filled with a fluid at a superatmospheric pressure;   wherein, when said drill assembly is being operated to impart an impact force to a drill bit, movement of said compressor piston toward said first chamber increases the pressure of the fluid in said first chamber, in said first passageway, and in said third chamber, thereby causing the movement of said hammer piston toward said fourth chamber; and   wherein, when said drill assembly is being operated to impart an impact force to a drill bit, movement of said compressor piston toward said second chamber increases the pressure of the fluid in said second chamber, in said second passageway, and in said fourth chamber, thereby causing the movement of said hammer piston toward said third chamber;   whereby a predetermined extent of movement of said hammer piston toward one of said third and fourth chambers can impart an impact force to a drill bit connected to said second end of said housing assembly while said drill assembly is being operated to impart an impact force to the drill bit.   
     
     
       13. A percussion drill assembly in accordance with claim 12, wherein said driver comprises: a rotary shaft rotatably mounted in said housing assembly and connected to said compressor piston such that rotation of said rotary shaft causes reciprocating movements of said compressor piston within said first compartment along the longitudinal axis of said first compartment, and   a motor positioned in said housing assembly and adapted to rotate said rotary shaft.   
     
     
       14. A percussion drill assembly in accordance with claim 13, wherein said motor has a liquid inlet and a liquid outlet, said motor has a stator and a rotor positioned between said liquid inlet and said liquid outlet, said rotor is connected to said rotary shaft so that rotation of said rotor causes corresponding rotation of said rotary shaft, and said liquid inlet of said motor is connected to a third passageway in said first end of said housing assembly so that liquid from a drill string flows through said third passageway and then flows between said stator and said rotor to said liquid outlet to effect rotation of said rotor with respect to said housing assembly, thereby rotating said rotary shaft and reciprocating said compressor piston. 
     
     
       15. A percussion drill assembly in accordance with claim 14, wherein one of said housing assembly and said motor has a bypass passageway therein in communication with said third passageway for passing a portion of liquid received from the drill string to said liquid outlet of said motor without said portion of the liquid going between said stator and said rotor. 
     
     
       16. A percussion drill assembly in accordance with claim 15, wherein said bypass passageway extends internally through said rotor. 
     
     
       17. A percussion drill assembly in accordance with claim 12, wherein said housing assembly comprises a bit adapter at said second end of said housing assembly for receiving a drill bit, said bit adapter having an anvil surface exposed to said second compartment; and wherein said drill assembly further comprises a drill bit removably connected to said bit adapter;   whereby a predetermined extent of movement of said compressor piston in one of its directions of movement causes sufficient movement of said hammer piston toward said anvil surface that said hammer piston strikes said anvil surface and imparts an impact blow to said bit adapter when said drill bit is in contact with a borehole bottom.   
     
     
       18. A percussion drill assembly in accordance with claim 17, wherein said bit adapter can slide axially with respect to the remainder of said housing assembly, whereby said bit adapter can move downwardly when the drill bit is not in contact with a borehole bottom; and wherein one of said first and second passageways is constructed such that fluid communication is established between said third and fourth chambers when said bit adapter moves downwardly as a result of the drill bit not being in contact with a borehole bottom.   
     
     
       19. A percussion drill assembly in accordance with claim 12, wherein said driver comprises: a rotary shaft rotatably mounted in said housing assembly,   a motor positioned in said housing assembly and adapted to rotate said rotary shaft, and   an oscillator element connecting said rotary shaft to said compressor piston such that rotation of said rotary shaft in a single direction causes reciprocating movements of said compressor piston within said first compartment along the longitudinal axis of said first compartment.   
     
     
       20. A percussion drill assembly in accordance with claim 12, wherein said driver comprises a rotary shaft rotatably mounted in said housing assembly, and a motor positioned in said housing assembly and adapted to rotate said rotary shaft; wherein said drill assembly further comprises a tubular member having a third passageway therethrough, said tubular member being positioned in said housing assembly and extending upwardly into and through said second compartment so that said third passageway in said tubular member is fluidly isolated from said second compartment;   wherein said compressor piston is a first annular piston positioned outwardly from and at least substantially concentrically with said rotary shaft;   wherein said hammer piston is a second annular piston positioned outwardly from and at least substantially concentrically with said tubular member for reciprocating movement of said second annular piston along said tubular member;   wherein said first end of said housing assembly has a fourth passageway for passage therethrough of liquid received from a drill string;   wherein said rotary shaft has a fifth passageway therethrough for the passage of liquid received from said fourth passageway to said third passageway in said tubular member; and   wherein said second end of said housing assembly has a sixth passageway therethrough for passage of liquid received from said third passageway.   
     
     
       21. A percussion drill assembly in accordance with claim 20, wherein said motor comprises a stator and a rotor; and wherein one of said housing assembly and said motor has a bypass passageway therein in communication with said fourth passageway for passing a portion of the liquid received from said drill string to said fifth passageway without said portion of the liquid going between said stator and said rotor.   
     
     
       22. A percussion drill assembly in accordance with claim 20, wherein said motor has a liquid inlet and a liquid outlet; wherein said motor has a stator and a rotor positioned between said liquid inlet and said liquid outlet;   wherein said rotor is connected to said rotary shaft via an upper coupling adapter, at least one universal joint, and a flow collar so that rotation of said rotor causes corresponding rotation of said rotary shaft;   wherein said liquid inlet of said motor is connected to said fourth passageway in said first end of said housing assembly so that liquid from said fourth passageway flows between said stator and said rotor to said liquid outlet to effect rotation of said rotor with respect to said housing assembly;   wherein one of said housing assembly and said motor has a bypass passageway therein in communication with said fourth passageway for passing a portion of the liquid received from said drill string to said fifth passageway without said portion of the liquid going between said stator and said rotor;   wherein said upper coupling adapter provides liquid communication between the bypass passageway and said liquid outlet of said motor; and   wherein said flow collar provides liquid communication between said liquid outlet of said motor and said fifth passageway in said rotary shaft;   whereby liquid from the drill string is passed through said motor, through said fifth passageway in said rotary shaft, through said third passageway in said tubular member, and through said sixth passageway in said second end of said housing assembly without entering either of said first and second chambers.   
     
     
       23. A percussion drill assembly in accordance with claim 22, wherein said bypass passageway extends internally through said rotor; wherein an end of said rotor which is adjacent to said first end of said housing assembly contains an inlet to said bypass passageway; and   wherein a choke is removably connected to said end of said rotor for varying a ratio of the amount of the liquid from said drill string which passes between said rotor and said stator to the amount of the liquid from said drill string which passes through said bypass passageway.   
     
     
       24. A percussion drill assembly in accordance with claim 12, wherein a first ring member is positioned between said compressor piston and the adjacent wall of said first compartment, said first ring member having a bleed passageway therethrough permitting a small flow of fluid between said first and second chambers, whereby fluid pressures in said first and second chambers can equalize when said compressor piston is stationary; and wherein a second ring member is positioned between said hammer piston and the adjacent wall of said second compartment, said second ring member having a bleed passageway therethrough permitting a small flow of fluid between said third and fourth chambers, whereby fluid pressures in said third and fourth chambers can equalize when said hammer piston is stationary.   
     
     
       25. A percussion drill assembly in accordance with claim 24, wherein said hammer piston provides a bleed passageway between said first and second passageways at the moment of impact by said hammer piston. 
     
     
       26. A percussion drill assembly in accordance with claim 12, wherein said driver comprises a rotary shaft rotatably mounted in said housing assembly, a motor positioned in said housing assembly and adapted to rotate said rotary shaft, and an oscillator element connecting said rotary shaft to said compressor piston such that rotation of said rotary shaft in a single direction causes reciprocating movements of said compressor piston within said first compartment along the longitudinal axis of said first compartment; wherein said motor has a liquid inlet and a liquid outlet; wherein said motor has a stator and a rotor positioned between said liquid inlet and said liquid outlet; wherein said rotor is connected to said rotary shaft so that rotation of said rotor causes corresponding rotation of said rotary shaft;   wherein said liquid inlet of said motor is connected to a third passageway in said first end of said housing assembly so that liquid from a drill string flows through said third passageway and then flows between said stator and said rotor to said liquid outlet to effect rotation of said rotor with respect to said housing assembly, thereby rotating said rotary shaft;   wherein said housing assembly comprises a bit adapter at said second end of said housing assembly for receiving a drill bit, said bit adapter having an anvil surface exposed to said second compartment;   wherein said bit adapter can slide axially with respect to the remainder of said housing assembly, whereby said bit adapter can move downwardly when the drill bit is not in contact with a borehole bottom;   whereby a predetermined extent of movement of said compressor piston in one of its directions of movement causes sufficient movement of said hammer piston toward said anvil surface that said hammer piston strikes said anvil surface and imparts an impact blow to said bit adapter when said drill bit is in contact with a borehole bottom; and   wherein one of said first and second passageways is constructed such that fluid communication is established between said third and fourth chambers when said bit adapter moves downwardly as a result of the drill bit not being in contact with a borehole bottom.   
     
     
       27. A percussion drill assembly in accordance with claim 26, further comprising a tubular member having a fourth passageway therethrough, said tubular member being positioned in said housing assembly and extending upwardly into and through said second compartment so that said fourth passageway in said tubular member is fluidly isolated from said second compartment; wherein said compressor piston is a first annular piston positioned outwardly from and at least substantially concentrically with said rotary shaft;   wherein said hammer piston is a second annular piston positioned outwardly from and at least substantially concentrically; with said tubular member for reciprocating movement of said second annular piston along said tubular member;   wherein said rotary shaft has a fifth passageway therethrough for the passage of liquid received from said third passageway to said fourth passageway in said tubular member; and   wherein said second end of said housing assembly has a sixth passageway therethrough for passage of liquid received from said fourth passageway.   
     
     
       28. A percussion drill assembly in accordance with claim 27, wherein a first ring member is positioned between said compressor piston and the adjacent wall of said first compartment, said first ring member having a bleed passageway therethrough permitting a small flow of fluid between said first and second chambers, whereby fluid pressures in said first and second chambers can equalize when said compressor piston is stationary; wherein a second ring member is positioned between said hammer piston and the adjacent wall of said second compartment, said second ring member having a bleed passageway therethrough permitting a small flow of fluid between said third and fourth chambers, whereby fluid pressures in said third and fourth chambers can equalize when said hammer piston is stationary; and   wherein said hammer piston provides a bleed passageway between said first and second passageways at the moment of impact by said hammer piston.   
     
     
       29. A liquid-driven, gas-operated, percussion drill assembly for drilling a borehole in a formation, said drill assembly comprising: an elongated housing assembly, said housing assembly having a first end and a second end opposite said first end, and a longitudinal axis extending from said first end to said second end;   an end portion of said housing assembly at said first end being adapted for removably connecting said drill assembly to a drill string, said end portion having a first passageway extending therethrough for the passing of a liquid received from the drill string;   an elongated first compartment formed within said housing assembly, said first compartment having a longitudinal axis which is at least generally parallel to the longitudinal axis of said housing assembly;   a first piston positioned within said first compartment for reciprocal motion within said first compartment along the longitudinal axis of said first compartment, said first piston dividing said first compartment into a first upper chamber and a first lower chamber which are substantially fluidly isolated from each other within said first compartment by the presence of said first piston;   a first shaft having a longitudinal axis, said first shaft being rotatably mounted in said housing assembly with the longitudinal axis of said first shaft being at least generally parallel to the longitudinal axis of said housing assembly, said first shaft being engaged with said first piston such that rotation of said first shaft causes reciprocating movements of said first piston within said first compartment along the longitudinal axis of said first compartment;   a motor positioned in said housing assembly and having a liquid inlet and a liquid outlet, said motor having a stator and a rotor positioned between said liquid inlet and said liquid outlet, said rotor being connected to said first shaft so that rotation of said rotor causes corresponding rotation of said first shaft, said liquid inlet of said motor being connected to the first passageway in said end portion of said housing assembly so that liquid from said first passageway flows between said stator and said rotor to said liquid outlet to effect rotation of said rotor with respect to said housing assembly, thereby rotating said first shaft and reciprocating said first piston;   an elongated second compartment formed within said housing assembly, said second compartment having a longitudinal axis which is at least generally parallel to the longitudinal axis of said housing assembly;   a second piston positioned within said second compartment for reciprocal motion within said second compartment along the longitudinal axis of said second compartment, said second piston dividing said second compartment into a second upper chamber and a second lower chamber which are substantially fluidly isolated from each other within said second compartment by the presence of said second piston;   a bit adapter having an anvil surface at a first end thereof and a drill bit receiving opening at a second end thereof, said bit adapter being removably attached to said second end of said housing assembly with said anvil surface of said bit adapter being exposed to said second lower chamber;   a second passageway providing fluid communication between said first upper chamber and a first one of said second upper chamber and said second lower chamber;   a third passageway providing fluid communication between said first lower chamber and a second one of said second upper chamber and said second lower chamber;   seals for sealing said first and second compartments and said second and third passageways from fluid communication with said first passageway, whereby said first and second compartments and said second and third passageways constitute a closed fluid system;   each of said first and second upper chambers, said first and second lower chambers, and said second and third passageways being filled with a gas at a superatmospheric pressure;   wherein movement of said first piston toward said first upper chamber compresses the gas in said first upper chamber and thus increases the pressure of the gas in said first upper chamber, in said second passageway, and in said first one of said second upper chamber and said second lower chamber, thereby causing the movement of said second piston toward said second one of said second upper chamber and said second lower chamber; and   wherein movement of said first piston toward said first lower chamber compresses the gas in said first lower chamber and thus increases the pressure of the gas in said first lower chamber, in said third passageway, and in said second one of said second upper chamber and said second lower chamber, thereby causing the movement of said second piston toward said first one of said second upper chamber and said second lower chamber.   
     
     
       30. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 29, further comprising a drill bit removably connected to said bit adapter, whereby a predetermined extent of movement of said first piston in one of its directions of movement causes sufficient movement of said second piston toward said anvil surface that said second piston strikes said anvil surface and imparts an impact blow to said bit adapter when said drill bit is in contact with a borehole bottom. 
     
     
       31. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 29, wherein said motor is positioned in said housing assembly between said end portion of said housing assembly and said first compartment, and wherein said second compartment is positioned in said housing assembly between said first compartment and said bit adapter. 
     
     
       32. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 29, further comprising an oscillator element connecting said first shaft to said first piston such that rotation of said first shaft in a single direction causes reciprocating movements of said first piston within said first compartment along the longitudinal axis of said first compartment. 
     
     
       33. A liquid-driven, gas-operated, percussion drill assembly in accordance with claims 29, wherein one of said housing assembly and said motor has a bypass passageway therein in communication with said first passageway for passing a portion of the liquid received from said drill string to said liquid outlet of said motor without said portion of the liquid going between said stator and said rotor. 
     
     
       34. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 29, wherein said bypass passageway extends internally through said rotor. 
     
     
       35. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 29, wherein said rotor is connected to said first shaft via at least one universal joint. 
     
     
       36. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 29, further comprising a second shaft positioned in said housing assembly and extending upwardly into said second compartment; wherein said second piston is a second annular piston positioned outwardly from and at least substantially concentrically with said second shaft for reciprocating movement of said second annular piston along said second shaft.   
     
     
       37. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 36, wherein said bit adapter has a passageway extending from the first end of said bit adapter to said second end of said bit adapter; wherein said second shaft has a passageway extending from an upper end of said second shaft to a lower end of said second shaft;   wherein said lower end of said second shaft is connected to said bit adapter so that the passageway in said second shaft is in fluid communication with the passageway in said bit adapter; and   wherein said second shaft extends all the way through said second compartment so that the passageway in said second shaft is fluidly isolated from said second compartment.   
     
     
       38. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 37, wherein said second shaft is mounted in said housing assembly so that it does not move relative to said housing assembly; wherein one of said housing assembly and said motor has a bypass passageway therein in communication with said first passageway for passing a portion of the liquid received from said drill string to a liquid outlet of said motor without said portion of the liquid going between said stator and said rotor;   wherein said first shaft has a passageway extending along the longitudinal axis of the first shaft;   wherein said rotor is connected to said first shaft via an upper coupling adapter, at least one universal joint and a flow collar;   wherein said upper coupling adapter provides liquid communication between the bypass passageway and said liquid outlet of said motor; and   wherein said flow collar provides liquid communication between said liquid outlet of said motor and the passageway in said first shaft;   whereby liquid from the drill string is passed through said motor, through the passageway in said first shaft, through the passageway in said second shaft, and through the passageway in said bit adapter without entering either of said first and second chambers.   
     
     
       39. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 38, wherein said bypass passageway extends internally through said rotor; wherein an end of said rotor which is adjacent to said end portion of said housing assembly contains an inlet to said bypass passageway; and   wherein a choke is removably connected to said end of said rotor for varying a ratio of the amount of the liquid from said drill string which passes between said rotor and said stator to the amount of the liquid from said drill string which passes through said bypass passageway.   
     
     
       40. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 29, wherein said second end of said housing assembly comprises an annular chuck positioned outwardly of and coaxially with an intermediate portion of said bit adapter, whereby said bit adapter can slide axially with respect to said chuck so that said bit adapter can move downwardly with respect to said chuck when the drill bit is not in contact with a borehole bottom. 
     
     
       41. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 40, further comprising: a plurality of drive pins engaging said chuck and said intermediate portion of said bit adapter so that rotation of said housing assembly by said drill string about the longitudinal axis of said housing assembly results in the rotation of said bit adapter about the longitudinal axis of said bit adapter, and   a retainer for retaining said bit adapter within said chuck.   
     
     
       42. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 41, wherein one of said second and third passageways is constructed such that fluid communication is established between said second upper chamber and said second lower chamber when said bit adapter moves downwardly as a result of the drill bit not being in contact with a borehole bottom. 
     
     
       43. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 29, wherein said housing assembly includes a connector segment positioned between said first compartment and said second compartment; wherein said second passageway extends through said connector segment from said first lower chamber to said second upper chamber;   wherein a first tubular member is mounted in an upper portion of said connector segment outward of and coaxially with said first shaft and slidably extends into a lower portion of said first piston so as to form a first annular passageway between said first tubular member and said first shaft and between said lower portion of said first piston and said first shaft;   wherein said first piston has a passageway extending from said first upper chamber to said first annular passageway;   wherein a second tubular member is mounted in a lower portion of said connector segment outward of and coaxially with said second shaft and slidably extends into an upper portion of said second piston so as to form a second annular passageway between said second tubular member and said second shaft and between said upper portion of said second piston and said second shaft;   wherein said connector segment has a passageway extending from said first annular passageway to said second annular passageway; and   wherein said second piston has a passageway extending from said second annular passageway to said second lower chamber;   whereby said third passageway comprises said passageway in said first piston, said passageway in said second piston, said first and second annular passageways, and the passageway in said connector segment which connects said first and second annular passageways.   
     
     
       44. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 43, wherein said second end of said housing assembly comprises an annular chuck positioned outwardly of and coaxially with an intermediate portion of said bit adapter, whereby said bit adapter can slide axially with respect to said chuck so that said bit adapter can move downwardly with respect to said chuck when the drill bit is not in contact with a borehole bottom; and wherein the length of said second tubular member is such that there is a clearance between a lower end of said second tubular member and an upper end of said second piston when said bit adapter moves downwardly as a result of the drill bit not being in contact with a borehole bottom;   such clearance establishing fluid communication between said second upper chamber and said second lower chamber, thereby preventing the raising of said second piston while said bit adapter has moved downwardly as a result of the drill bit not being in contact with a borehole bottom; and   said length of said second tubular member being such that said clearance is eliminated when said bit adapter moves upwardly as a result of the drill bit being in contact with a borehole bottom.   
     
     
       45. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 29, wherein a first ring member is positioned between said first piston and the adjacent wall of said first compartment, said first ring member having a bleed passageway therethrough permitting a small flow of gas between said first upper chamber and said first lower chamber, whereby gas pressures in said first upper chamber and said first lower chamber can equalize when said first piston is stationary; and wherein a second ring member is positioned between said second piston and the adjacent wall of said second compartment, said second ring member having a bleed passageway therethrough permitting a small flow of gas between said second upper chamber and said second lower chamber, whereby gas pressures in said second upper chamber and said second lower chamber can equalize when said second piston is stationary.   
     
     
       46. A liquid-driven, gas-operated, percussion drill assembly in accordance with claim 45, wherein said second piston provides a bleed passageway between said second and third passageways at the moment of impact by said second piston. 
     
     
       47. A percussion drill assembly for drilling a borehole in a formation, said drill assembly comprising: an elongated housing assembly having a first end adapted to removably connect said drill assembly to a drill string, and a second end adapted to receive a drill bit;   a first compartment formed within said housing assembly and having a longitudinal axis;   a rotary shaft rotatably mounted in said housing assembly and extending into said first compartment;   a motor positioned in said housing assembly and adapted to rotate said rotary shaft;   an annular compressor piston having an inner annular wall, said annular compressor piston being positioned outwardly from and at least substantially concentrically with said rotary shaft positioned within said first compartment for reciprocating motion of said annular compressor piston within said first compartment along the longitudinal axis of said first compartment in response to rotation of said rotary shaft, said annular compressor piston dividing said first compartment into a first chamber and a second chamber which are substantially fluidly isolated from each other within said first compartment by the presence of said annular compressor piston;   a second compartment formed within said housing assembly and having a longitudinal axis;   a hammer piston positioned within said second compartment for reciprocating motion within said second compartment along the longitudinal axis of said second compartment, said hammer piston dividing said second compartment into a third chamber and a fourth chamber which are substantially fluidly isolated from each other within said second compartment by the presence of said hammer piston, said hammer piston having a working area;   a first passageway providing fluid communication between said first chamber and said third chamber;   a second passageway providing fluid communication between said second chamber and said fourth chamber;   wherein a hammer piston drive volume comprises one of said third and fourth chambers and one of said first and second passageways while a hammer piston return volume comprises the other of said third and fourth chambers and the other one of said first and second passageways;   seals for sealing said first and second compartments and said first and second passageways from fluid communication with any fluid received from the drill string, whereby said first and second compartments and said first and second passageways constitute a closed fluid system;   each of said first, second, third, and fourth chambers, and said first and second passageways being precharged with a fluid at a superatmospheric pressure, wherein said superatmospheric pressure has a value such that said hammer piston is reciprocated within ±20% of a natural resonant frequency for said drill assembly when said rotary shaft is rotated at a design value of rpm;   wherein, when said drill assembly is being operated to impart an impact force to a drill bit, movement of said compressor piston toward said first chamber increases the pressure of the fluid in said first chamber, in said first passageway, and in said third chamber, thereby causing the movement of said hammer piston toward said fourth chamber; and   wherein, when said drill assembly is being operated to impart an impact force to a drill bit, movement of said compressor piston toward said second chamber increases the pressure of the fluid in said second chamber, in said second passageway, and in said fourth chamber, thereby causing the movement of said hammer piston toward said third chamber;   whereby a predetermined extent of movement of said hammer piston toward one of said third and fourth chambers can impart an impact force to a drill bit connected to said second end of said housing assembly while said drill assembly is being operated to impart an impact force to the drill bit.   
     
     
       48. A percussion drill assembly in accordance with claim 47, wherein said value of superatmospheric pressure is selected in view of said working area of said hammer piston, said hammer piston return volume, and said hammer piston drive volume so that said hammer piston is reciprocated within ±20% of a natural resonant frequency for said drill assembly when said rotary shaft is rotated at a design value of rpm. 
     
     
       49. A percussion drill assembly in accordance with claim 47, wherein said value of superatmospheric pressure is selected in view of said working area of said hammer piston, said hammer piston return volume, and said hammer piston drive volume so that said hammer piston is reciprocated within ±10% of a natural resonant frequency for said drill assembly when said rotary shaft is rotated at a design value of rpm. 
     
     
       50. A method of operating a percussion drill assembly for drilling a borehole in a formation, wherein said drill assembly comprises: an elongated housing assembly having a first end adapted for connecting said drill assembly to a drill string, and a second end adapted to receive a drill bit;   a first compartment formed within said housing assembly and having a longitudinal axis;   a rotary shaft rotatably mounted in said housing assembly and extending into said first compartment;   a motor positioned in said housing assembly and adapted to rotate said rotary shaft;   an annular compressor piston having an inner annular wall, said annular compressor piston being positioned outwardly from and at least substantially concentrically with said rotary shaft positioned within said first compartment for reciprocating motion of said annular compressor piston within said first compartment along the longitudinal axis of said first compartment in response to rotation of said rotary shaft, said annular compressor piston dividing said first compartment into a first chamber and a second chamber which are substantially fluidly isolated from each other within said first compartment by the presence of said annular compressor piston;   a second compartment formed within said housing assembly and having a longitudinal axis;   a hammer piston positioned within said second compartment for reciprocating motion within said second compartment along the longitudinal axis of said second compartment, said hammer piston dividing said second compartment into a third chamber and a fourth chamber which are substantially fluidly isolated from each other within said second compartment by the presence of said hammer piston, said hammer piston having a working area;   a first passageway providing fluid communication between said first chamber and said third chamber;   a second passageway providing fluid communication between said second chamber and said fourth chamber;   wherein a hammer piston drive volume comprises one of said third and fourth chambers and one of said first and second passageways while a hammer piston return volume comprises the other of said third and fourth chambers and the other one of said first and second passageways; and   seals for sealing said first and second compartments and said first and second passageways from fluid communication with any fluid received from the drill string, whereby said first and second compartments and said first and second passageways constitute a closed fluid system; said method comprising:     precharging said closed fluid system with a fluid at a superatmospheric pressure;   connecting said first end of said drill assembly to a drill string;   connecting said second end of said drill assembly to a drill bit;   operating said drill assembly to impart an impact force to said drill bit by actuating said motor to rotate said shaft and thereby reciprocate said compressor piston so that movement of said compressor piston toward said first chamber increases the pressure of the fluid in said first chamber, in said first passageway, and in said third chamber, thereby causing the movement of said hammer piston toward said fourth chamber; and so that movement of said compressor piston toward said second chamber increases the pressure of the fluid in said second chamber, in said second passageway, and in said fourth chamber, thereby causing the movement of said hammer piston toward said third chamber;   whereby a predetermined extent of movement of said hammer piston toward one of said third and fourth chambers imparts an impact force to said drill bit;   wherein said motor rotates said rotary shaft at a rate which causes said hammer piston to reciprocate at a frequency which is within ±20% of a natural resonant frequency for said drill assembly.   
     
     
       51. A method in accordance with claim 50, wherein said motor rotates said rotary shaft at a rate which causes said hammer piston to reciprocate at a frequency which is within ±10% of a natural resonant frequency for said drill assembly. 
     
     
       52. A method in accordance with claim 50, further comprising rotating the drill string to thereby rotate said drill bit. 
     
     
       53. A method in accordance with claim 50, further passing drilling mud through the drill string into and through said motor to actuate said motor and then passing the drilling mud from said motor to and through said drill bit to flush drilling debris from the drill bit. 
     
     
       54. A method in accordance with claim 50, wherein the fluid which is precharged to said closed fluid system is a gas. 
     
     
       55. A method in accordance with claim 50, further comprising selecting said superatmospheric pressure in view of said working area of said hammer piston, said hammer piston return volume, and said hammer piston drive volume so that said hammer piston is reciprocated within ±10% of a natural resonant frequency for said drill assembly when said rotary shaft is rotated at a design value of rpm. 
     
     
       56. A percussion hammer assembly for imparting an impact to a driven element, said hammer assembly comprising: an elongated housing assembly having a first end adapted to removably connect said hammer assembly to a support device, and a second end adapted to receive a driven element;   a hammer piston compartment formed within said housing assembly and having a longitudinal axis;   a hammer piston positioned within said hammer piston compartment for reciprocal motion within said hammer piston compartment along the longitudinal axis of said hammer piston compartment, said hammer piston dividing said hammer piston compartment into a first chamber and a second chamber which are substantially fluidly isolated from each other within said hammer piston compartment by the presence of said hammer piston;   a fluid compressor positioned within said housing assembly and having a first port in said first chamber and a second port in said second chamber, whereby said compressor and said hammer piston compartment form a compressor fluid system;   a driver mounted in said housing assembly and connected to said compressor so as to drive said compressor to produce a high fluid pressure in said first port and a low fluid pressure in said second port during a first half cycle of operation of said compressor and to produce a low fluid pressure in said first port and a high fluid pressure in said second port during a second half cycle of operation of said compressor;   seals for sealing said compressor fluid system from fluid communication with any fluid received from outside said elongated housing assembly during operation of said hammer assembly, whereby said compressor fluid system is a closed fluid system;   wherein, when said hammer assembly is being operated to impart an impact force to said driven element, a high fluid pressure in said first chamber and a low fluid pressure in said second chamber causes a movement of said hammer piston toward said second chamber; and   wherein, when said hammer assembly is being operated to impart an impact force to said driven element, a low fluid pressure in said first chamber and a high fluid pressure in said second chamber causes a movement of said hammer piston toward said first chamber;   whereby a predetermined extent of movement of said hammer piston toward one of said first and second chambers can impart an impact force to said driven element at said second end of said housing assembly while said hammer assembly is being operated for the purpose of imparting an impact force to said driven element.   
     
     
       57. A percussion hammer assembly in accordance with claim 56, wherein said hammer piston is a free-floating piston. 
     
     
       58. A percussion hammer assembly in accordance with claim 56, wherein said driver comprises a fluid motor which is driven by a driving fluid which is supplied into the elongated housing assembly during operation of said hammer assembly, and wherein the driving fluid is exhausted from said fluid motor through said housing assembly. 
     
     
       59. A percussion hammer assembly in accordance with claim 56, wherein said housing assembly comprises an adapter at said second end of said housing assembly for receiving said driven element so that said driven element has an anvil surface exposed to said hammer piston compartment; and wherein said hammer assembly further comprises said driven element removably connected to said adapter;   whereby a predetermined extent of movement of said hammer piston in one of its directions of movement causes said hammer piston to strike said anvil surface and impart an impact blow to said driven element when said driven element is in contact with a surface located below said driven element.   
     
     
       60. A percussion hammer assembly in accordance with claim 59, wherein said adapter can slide axially with respect to the remainder of said housing assembly, whereby said adapter can move downwardly when the driven element is not in contact with a surface located below said driven element; and wherein said housing assembly has a connector passageway which is blocked when said driven element is in contact with a surface located below said driven element and which is unblocked when said driven element moves downwardly as a result of the driven element not being in contact with a surface located below said driven element, whereby said connector passageway provides fluid communication between said first and second chambers when said connector passageway is unblocked.   
     
     
       61. A percussion hammer assembly in accordance with claim 56, wherein said driver comprises a motor having a liquid inlet and a liquid outlet, wherein said motor has a stator and a rotor positioned between said liquid inlet and said liquid outlet, wherein said rotor is connected to a rotary shaft so that rotation of said rotor causes corresponding rotation of said rotary shaft, wherein the rotation of said rotary shaft drives said compressor, and wherein said liquid inlet of said motor is connected to an inlet passageway in said first end of said housing assembly so that liquid flows through said inlet passageway and then flows between said stator and said rotor to said liquid outlet to effect rotation of said rotor with respect to said housing assembly, thereby rotating said rotary shaft and driving said compressor. 
     
     
       62. A percussion hammer assembly in accordance with claim 56, wherein a compressor compartment is formed within said housing assembly, said compressor compartment having a longitudinal axis; wherein said compressor comprises a compressor piston positioned within the compressor compartment for reciprocal motion within said compressor compartment along the longitudinal axis of said compressor compartment, said compressor piston dividing said compressor compartment into a third chamber and a fourth chamber which are substantially fluidly isolated from each other within said compressor compartment by the presence of said compressor piston;   wherein said first port provides fluid communication with said third chamber, and said second port provides fluid communication with said fourth chamber; and   wherein said driver is connected to said compressor piston to cause reciprocating movements of said compressor piston within said compressor compartment along the longitudinal axis of said compressor compartment.   
     
     
       63. A percussion hammer assembly in accordance with claim 62, wherein said driver comprises: a rotary shaft rotatably mounted in said housing assembly and connected to said compressor piston such that rotation of said rotary shaft causes reciprocating movements of said compressor piston within said compressor compartment along the longitudinal axis of said compressor compartment, and   a motor positioned in said housing assembly and adapted to rotate said rotary shaft.   
     
     
       64. A percussion hammer assembly in accordance with claim 62, wherein said driver comprises: a rotary shaft rotatably mounted in said housing assembly,   a motor positioned in said housing assembly and adapted to rotate said rotary shaft, and   an oscillator element connecting said rotary shaft to said compressor piston such that rotation of said rotary shaft in a single direction causes reciprocating movements of said compressor piston within said compressor compartment along the longitudinal axis of said compressor compartment.   
     
     
       65. A percussion hammer assembly in accordance with claim 62, wherein said driver comprises a rotary shaft rotatably mounted in said housing assembly, and a motor positioned in said housing assembly and adapted to rotate said rotary shaft; wherein said hammer assembly further comprises a tubular member having a passageway therethrough, said tubular member being positioned in said housing assembly and extending upwardly into and through said hammer piston compartment so that the passageway in said tubular member is fluidly isolated from said hammer piston compartment;   wherein said compressor piston is a first annular piston positioned outwardly from and at least substantially concentrically with said rotary shaft;   wherein said hammer piston is a second annular piston positioned outwardly from and at least substantially concentrically with said tubular member for reciprocating movement of said second annular piston along said tubular member.